In a wavelength division multiplexing (WDM) optical transmission system, reconfigurable optical add-drop multiplexer (ROADM) apparatuses, each having a function for adding/dropping wavelengths, are connected in multiple stages to achieve long-distance transmission. In order to reduce variations (for example, tilts) among optical power levels for respective wavelengths which are monitored by the ROADM apparatus at a receiving side, a wavelength selective switch (WSS) in the ROADM apparatus at a transmitting side controls the optical power levels for the respective wavelengths to thereby ensure favorable transmission characteristics. Such control is called “pre-emphasis control”.
The ROADM apparatuses are expensive since they include high-cost elements, such as WSSs, in order to implement the add/drop functions. Accordingly, a relay apparatus (an in-line amplifier (ILA)) having only functions for amplifying signals and transmitting the signals is installed in place of a ROADM apparatus at a place where no wavelength add/drop is involved, to thereby construct a network at low cost (see, for example, Japanese Laid-open Patent Publication No. 2000-332686).
However, when an ILA is installed in place of a ROADM apparatus, an optical fiber length between the ROADM apparatuses increases and the span loss also increases. As the span loss increases, a tilt resulting from wavelength dependent loss (WDL) increases. When the magnitude of the tilt exceeds the amount of pre-emphasis control that can be performed by a WSS, it is difficult to install an ILA.